1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a layout of an electrostatic protection element.
2. Description of Related Art
A driver IC for driving a display panel is required to be protected from electrostatic discharge. To realize such protection from electrostatic discharge, for example, Japanese Patent Application Laid-Open Publication No. 2005-252214 discloses a technique for providing an electrostatic protection element in such a driver IC.
Japanese Patent Application Laid-Open Publication No. 2005-072607 also discloses a layout of an electrostatic protection element to be employed for a semiconductor device. The semiconductor device described in this document disposes an electrostatic protection element for each input-output pad.
In the case of the layout configuration described in the JP-A No. 2005-072607, however, the electrostatic protection element area increases if an electrostatic protection element is provided for each input-output pad, for example, in a PDP (plasma display panel) driver IC that includes many input-output pads. As a result, the chip size also increases and the semiconductor device cost increases.
In each of those PDP driver ICs, therefore, a GND-VDD (ground potential-power source Vdd potential) protection element common to a plurality of input-output pads is disposed in the protection element regions 101 at both sides of the subject chip respectively as shown in a layout in FIG. 1. Here, a circuit cell disposed in each output pad shown in FIG. 1 and used for controlling output signals is an output circuit and the output circuit is configured by, for example, a circuit element according to the Push-Pull method. Hereunder, there will be described an output circuit disposed region as an output circuit unit.
FIG. 2 shows a circuit corresponding to the layout shown in FIG. 1. In FIG. 2, each output circuit of the output circuit unit 100 shown in FIG. 1 is substituted for a basic inverter. Rn and Rp are line resistors disposed between each of GND and VDD of the output circuits and a protection element. A protection diode D1 is formed in a protection element region 101.
In order to improve the resistance to electrostatic discharge, it is important to avoid concentration of the electrostatic discharge current at a specific element.
FIG. 3A shows an example of a current path formed when electrostatic discharge is applied to an output terminal OUT with reference to the GND.
As shown clearly in FIG. 3A, there are resistors Rn and Rp disposed on lines from the output circuits GND and VDD to the anode and cathode of the protection element D1. The current path 1 does not go through the electrostatic protection element D1, so that the electrostatic discharge current flow is concentrated at the Nch transistor N1 of an output circuit. The current path 2 is provided to reduce the electrostatic discharge load applied to the Nch transistor N1 through the current path 1 by distributing the load. A Pch transistor P1 is provided on this current path 2.
In order to improve the resistance to electrostatic discharge, it is important to reduce the values of the line resistors Rn and Rp shown in FIG. 3A. Large values of the line resistors Rn and Rp reduce the current flow to the current path 2, thereby lowering the effect of the current path 2 including the electrostatic protection element D1.
FIG. 3B shows an example of a current path formed when electrostatic discharge is applied to the output terminal OUT with reference to the VDD. Similarly to FIG. 3A, it is also important to reduce the values of the line resistors Rn and Rp in FIG. 3B.
In case where an electrostatic protection element D1 is disposed at both sides of a driver IC as shown in FIGS. 1 and 2 according to the conventional technique, the line resistance is not equalized between the output circuit of each input-output pad and the electrostatic protection element D1. For example, particularly in an output circuit having a large resistance value in a line up to the electrostatic protection element D1, it is impossible to obtain an electrostatic discharge effect for the electrostatic protection element D1. This is why a large resistance value appears in the current path 2 of the center output pad in the subject layout, which is farthest from any of the electrostatic protection elements D1, thereby an electrostatic discharge current is concentrated at the current path 1 and the resistance to electrostatic discharge is minimized. This has been a problem.